Effects of irrigation water quality and soil type on the soil and plant associated microbiome, abundance, diversity and transferability of antibiotic resistance genes in Gram-negative bacteria

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The reuse of wastewater for irrigation in agriculture is an efficient way of saving water and increasing food production for a growing population under conditions of climate change. Investments in infrastructure cause a shift from irrigation with untreated wastewater to irrigation with treated wastewater in many countries. SP 5 of the proposed research unit will contribute to testing the hypotheses that i) environmental concentrations of pollutants released from soil and taken up by plants are high enough to select antibiotic resistance and trigger horizontal gene transfer in soils and plants and ii) the soil type modulates the release of pollutants and the associated selection of antibiotic resistance.The effect of the addition of treated or untreated wastewater to Leptosols, Phaeozems and Vertisols, that have been irrigated with untreated wastewater for >80 years, on the microbial community composition and the abundance of antibiotic resistance genes (ARGs) as well as mobile genetic elements (MGEs) associated with Gram-negative bacteria will be tested in a joint incubation and batch experiment based on total community DNA. For a subset of soil samples horizontal gene transfer (HGT) rates among Gram-negative bacteria will be determined. Isolated Enterobacteria (SP 6) will be screened for the presence of transferable plasmids. The mobilization of ARGs to InCP-1 plasmids due to selection by antibiotics and disinfectants added with wastewater and released from soil will be tested in a satellite experiment, in which the soil bacterium Acinetobacter baylyi BD413 carrying IncP-1 plasmids without antibiotic resistance genes will be applied to the differently treated soils, re-isolated after 28 days and screened for acquired ARGs by IncP-1 plasmids.The relevance of plants for the selection and spreading of ARGs and the transfer into the food chain will be investigated in the joint soil column experiment with monolithic “undisturbed” soil columns planted with cilantro (Coriandrum sativum). SP 5 will assess the microbial community composition in phyllosphere and roots/rhizosphere, the relative abundance of ARGs and MGEs of Gram-negative bacteria, and HGT-rates among Gram-negative bacteria.SP 5 contributes the same expertise and techniques to the joint field experiment with Phaeozem soil irrigated with untreated wastewater for >80 years and planted with cilantro in order to bridge controlled laboratory and greenhouse experiments to real world conditions, especially regarding the effects of irrigation water quality on phyllosphere bacteria, which are difficult to study under greenhouse conditions (reduced radiation and dust inputs).By linking its results and expertise to the data and knowledge of the other SPs of the RU also by means of the integrated mathematical model (SP 7), SP 5 contributes to a mechanistic understanding of interactions between pollutants, antibiotic resistance and pathogens in changing wastewater irrigation systems.